The legacy YCbCr color format is a color-opponent, non-constant luminance format, where signals are interpreted based on color differences in an opposing manner. In YCbCr and similar color-opponent formats (such as YUV) the goal is to separate luma from chroma information for the purposes of chroma subsampling (i.e., 4:2:2 and 4:2:0). Chroma sub-sampling reduces the amount of data required to represent an image without affecting perceptually overall picture quality. Separating color from luma has also been proven to yield coding advantages in a variety of image and video coding standards, such as JPEG, MPEG-1, MPEG-2, AVC, HEVC, and the like.
Recently, High dynamic range (HDR) and wide color gamut (WCG) content have revealed the limitations of existing color encoding methods. Errors that were previously small with standard dynamic range can become magnified. Rec. ITU-R BT.2100-0, “Image parameter values for high dynamic range television for use in production and international programme exchange,” ITU, July, 2016, which is incorporated herein by reference in its entirety, provides an alternative method for color difference encoding using a new color imaging format, referred to as ICtCp (or ICTCP).
Like YCbCr, ICtCp is a color-opponent based encoding scheme intended to separate luma from chroma information. In addition, ICtCp offers constant intensity (CI) representation. The CI neutral (grey) axis is encoded with the SMPTE ST 2084 or Hybrid Log-Gamma (HLG) non-linearity functions to match the human visual system, and to optimize it for high dynamic range signal encoding. Starting from RGB or XYZ representations, color transformation matrices to the ICtCp color format have been optimized for the human visual system perception of HDR and WCG content.
A reference electro-optical transfer function (EOTF) for a given display characterizes the relationship between color values (e.g., luminance) of an input video signal to output screen color values (e.g., screen luminance) produced by the display. For example, ITU Rec. BT. 1886, “Reference electro-optical transfer function for flat panel displays used in HDTV studio production,” ITU, March 2011, defines the reference EOTF for flat panel displays based on measured characteristics of the Cathode Ray Tube (CRT). Given a video stream, information about its EOTF or inverse EOTF or OETF is typically embedded in the bit stream as metadata (e.g., video usability information (VUI) metadata).
Most consumer desktop displays currently support luminance of 200 to 300 cd/m2 or nits. Most consumer HDTVs range from 300 to 500 nits with new models reaching 1000 nits (cd/m2). Such conventional displays thus typify a lower dynamic range (LDR), also referred to as a standard dynamic range (SDR), in relation to HDR. As the availability of HDR content grows due to advances in both capture equipment (e.g., cameras) and HDR displays (e.g., the PRM-4200 professional reference monitor from Dolby Laboratories), HDR content may be color graded and displayed on HDR displays that support higher dynamic ranges (e.g., from 1,000 nits to 5,000 nits or more). Such displays may be defined using alternative EOTFs that support high luminance capability (e.g., 0 to 10,000 nits). An example of such an EOTF is defined in SMPTE ST 2084:2014, “High Dynamic Range EOTF of Mastering Reference Displays,” SMPTE, 2014, which is incorporated herein by reference, and BT. 2100. Signals encoded using SMPTE ST 2084 may also be referred to as being “PQ-coded,” to distinguish them from traditional signals which were “gamma-coded.”
Most of the existing video compression standards, such as MPEG-1, MPEG-2, AVC, HEVC, and the like, have been tested, evaluated, and optimized for gamma-coded images in the YCbCr color space using the BT. 709 or BT. 2020 containers; however, experimental results have shown that the ICtCp color format may provide a better representation for high-dynamic range images with 10 or more bits per pixel per color component. In addition, SMPTE ST 2084 and HLG-based encoding provide far more efficient encoding for high dynamic range images than traditional gamma-based encoding. To improve existing coding standards, such as HEVC, as appreciated by the inventors here, improved techniques for the coding of video represented in multiple color imaging formats are needed.
As used herein, the term “color format” relates to a representation of a video signal using at least a color space, such as YCbCr, ICtCp, and the like, and a display-related encoding (also referred to as “transfer characteristics,”), such as linear, gamma, PQ, and the like. The term may also relate to a color gamut representation, such as those defined by BT. 709, BT. 2020, and the like, and a chroma sub-sampling format (e.g., 4:4:4, 4:2:2, or 4:2:0).
As used herein, the term “metadata” relates to any auxiliary information that is transmitted as part of the coded bitstream and assists a decoder to render a decoded image. Such metadata may include, but are not limited to, color space or gamut information, reference display parameters, and auxiliary signal parameters, as those described herein.
The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section. Similarly, issues identified with respect to one or more approaches should not assume to have been recognized in any prior art on the basis of this section, unless otherwise indicated.